This invention relates generally to fluid processing, and in particular to distillation processes for separating fluids of different volatility. In particular, the invention concerns detection of flooding conditions in a distillation column.
Distillation and related processes are used to separate components of a fluid mixture. Distillation is performed by heating the mixture to a boiling point, and extracting products with relatively higher or lower volatility based on differences in boiling temperature, condensation point, and vapor pressure.
Distillation equipment and techniques vary with application. In alcoholic beverage production and other processes, for example, the distillation apparatus is typically referred to a still. In a simple batch still, fermentation products such as mash are boiled in a single-pass process in order produce a distillate with mixed composition, including both water and alcohol as well as oils and other flavor components of the mash. The pressure, temperature and boiling rate are carefully controlled in order to regulate flavor and alcohol content.
In industrial techniques it is often necessary to more completely separate the final products, and the batch process is repeated a number of times based on the products' different boiling points and the desired level of purity. In continuous distillation, on the other hand, the fluid mixture or “feedstock” is constantly fed into a boiler to generate continuous vapor flow through the distillation column, with the more volatile distillate (or top product) being removed near the top of the column and the less volatile residue (or “bottoms”) removed near the bottom.
In fractional distillation the heating vessel is typically described as a reboiler, and the distillation column is divided into a bottom stripper section and a top rectifier section, with a feedstock inlet between the stripper and the rectifier. The bottom product is continuously recycled through the stripper via the reboiler, in order to increase the heavy fraction of the bottoms. Similarly, the top product is continuously recycled through the rectifier section via a reflux drum, in order to increase the light fraction of the distillate. The striper and rectifier sections also typically include distillation trays or a packing material to provide increased surface area for condensation, reheating and vaporization. This increases mass transfer and throughput, improving efficiency and providing more refined, higher-purity top and bottom products.
In petroleum refining, the process feed (e.g., crude oil) often has multiple components, and large distillation columns (or distillation towers) are used to extract a variety of different products (or fractions) at different heights. In natural gas processing (or “sweetening”) and emissions control applications, on the other hand, the goal is to remove unwanted impurities and the distillation column is often referred to as an absorption column or scrubber.
Distillation is also used in air separation and other cryogenic processes, for example to separate liquid oxygen, liquid nitrogen, and liquid argon, and for a variety of carbon dioxide-based processes. Other techniques include steam distillation for heat-sensitive and high-boiling point materials, and vacuum distillation for fluids that react with oxygen or nitrogen in the air, or are highly sensitive to environmental contaminants.
In each of these applications, safety and efficiency demand a high degree of process control. While the output rate is ultimately determined by thermal input, moreover, there is a constant tradeoff between heating rate and the risk of flooding. Flooding occurs when the vapor flow disrupts the condensation flow, causing axial mixing that reduces differentiation and decreases efficiency. When unchecked, flooding can also cause a runaway condition that disrupts the entire distillation process, resulting in product losses and system downtime. Thus there is a constant need for improved flooding detection and control techniques, particularly techniques that are adaptable to a range of different distillation systems and processes.